In general, this invention relates to camera exposure control systems. More particularly, it relates to such a control system for automatically changing the setting of a diaphragm opening from a manual presetting thereof in a camera having a shutter operating in an automatic manner for controlling exposure time.
It is a well known principle of photography that a plurality of different combinations of shutter speed and aperture setting result in the same amount of light impinging on the film. For example, the combination of an exposure time of 1/30th of a second and an aperture setting of f4 results in the same amount of light impinging on the film as the combination of an exposure time of 1/60th of a second and an aperture setting of f2.8. The foregoing example is a specific case of the general rule that total exposure is a function of the product of the two variables, light intensity and exposure time. Thus, a geometric increase in one of these variables, such as doubling, accompanied by a corresponding geometric decrease in the other variable, such as halving, has no effect on the total amount of light impinging on the film.
The foregoing does not mean that the resulting photographs taken with two such combinations will in all respects be the same. To the contrary, there can be significant differences between them. Firstly, the depth of field is affected by the aperture. In particular, objects in the foreground of a scene which will be acceptably in focus when a relatively small aperture is used will be out of focus when a relatively large aperture is used. Secondly, movements, either of objects in the scene or of the camera, have different effects depending upon the shutter speed used. In particular, it is generally true that a photographer while hand-holding a camera will inadvertently move it such that exposure time as prolonged as about 1/30th of a second result in blurred photographs. Another factor relevant here is that the recently developed fast electric shutters for expensive cameras have a maximum shutter speed, this typically being 1/1000th of a second. Circumstances arise in which this presents a problem. For example, a photographer may be taking a picture of a very brightly lighted scene with a manually preset aperture at f2. In these kinds of circumstances, the shutter speed necessary to prevent overexposure may exceed the maximum end of the adjustability range of the shutter speed.
A number of prior art approaches have been directed to these problems but have not provided a satisfactory solution. Generally, these prior art approaches are characterized by means for providing a warning indication to the photographer. According to one such prior art approach a display member visible through the viewfinder is lighted under circumstances in which the scene is so bright as to result in overexposure with the particular preset aperture even with the shutter speed being at the fastest speed in the range. A warning of a similar nature is sometimes given by locking the shutter release. Other circumstances in which such a warning has been given include the situation in which the scene is so dimly lighted that, with the particular preset aperture, the exposure time necessary to afford adequate exposure exceeds the threshold at which hand trembling will likely blur the resulting photograph. These warning systems are unsatisfactory for a number of reasons. First of all, the photographer must notice the warning and then take corrective steps by manually adjusting the lens aperture. This of course is bothersome and time consuming. Owing to the delay incident to doing this, it is possible that the photographer may miss the picture he wanted to take.